Gradiently varied chain packing/orientation states of polymer thin films revealed by polarization-dependent infrared absorption

• Gradiently varied packing/orientation states were discovered for polymer thin films on the gold substrates.
• Polarization-dependent band splitting was correlated to evolution of the chain packing/orientation state.
• A new experimental scheme using p-polarized light was developed to adjust the field vector direction for infrared absorption.

Gradiently varied chain packing/orientation states of the polyacrylamide (PAL) thin films spin-coated on the gold (Au) substrates were found via the polarized reflection–absorption Fourier transform infrared spectroscopy (RA-FTIR). As the film thickness increases, the splitted amide I bands provide a direct evidence that the PAL thin films are of a gradiently varied bi-layered structure. In the bottom layer, most of the PAL molecules show random orientation which is induced by the non-favorable interaction from the adjacent Au surface. In the top layer, most of PAL molecules show parallel orientation to the Au surface which is induced by spin coating, evidenced by the enhanced low-frequency splitted amide I band (∼1658 cm−1) and N–H stretching modes of the amino groups when the light electric field vector is adjusted to be parallel to the Au surface. The observation reported in this study should be of universal significance for polymer thin films on the supported substrates, where the interfacial interaction as well as spin coating could vary the polymer packing/orientation states substantially.

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